1. Field of the Invention
The present invention relates to a method and apparatus for bending a sheet metal member through application of pressure thereon.
2. Description of the Related Art
Conventionally, components obtained by bending a sheet metal member have been used in various fields such as a rail used in a copying machine, a printer, or the like. FIG. 11A is a cross-sectional view illustrating a generally-used bending apparatus which is in a state before bending is started. FIG. 11A illustrates a sheet metal member (work) W which is to be subjected to bending, and the generally-used bending apparatus including a pair of dies 101a and 101b, a punch 102, and a knock out die 103. The work W is placed on the pair of dies 101a and 101b so as to bridge the dies 101a and 101b. The knock out die 103 is placed between the dies 101a and 101b below the work W, and the punch 102 is placed at a position at which the punch 102 is opposed to the knock out die 103 through the work W. The knock out die 103 is a member serving as a bracket when the work W is pressurized by the punch 102, and is pressed against the work W from below by a spring member (not shown). As the punch 102 descends (relatively moves) with respect to the dies 101a and 101b and the work W is bent, the spring member contracts, whereby the knock out die 103 descends. As a result, the work W is bent into a U-shape. FIG. 11B illustrates the work W which is bent into the U-shape.
In general, highly-accurate flatness and straightness in a bending ridge line direction are required for a bending member used in a precision apparatus. However, during the bending, a concave surface (on a punch side) and a convex surface (on a knock out die side) of the work W are applied with a compressive stress and a tensile stress, respectively. Accordingly, the work W cambers in a direction indicated by an arrow A of FIG. 11B due to a residual stress as a result of the bending.
In order to suppress a generation of the camber as described above, Japanese Patent No. 3280733 describes a structure in which a female mold (knock out die) is divided into multiple pieces in a direction parallel to a bending ridge line. With the structure as described above, the respective divided female molds can move in accordance with a frictional force generated between the work W and the female molds during the bending in the direction parallel to the bending ridge line. Accordingly, processing can be performed while releasing a stress generated during the bending in the direction parallel to the bending ridge line, with the result that the camber of the work can be suppressed.
Further, Japanese Patent Application Laid-Open No. 2004-074239 describes a method of forming multiple locally-deformed portions (concave portions) on a female mold (knock out die) side of a work simultaneously with the bending. With the structure as described above, a tensile stress applied on the knock out die side of the work is partially compensated by a compressive stress which is applied through the formation of the locally-deformed portions, to thereby suppress the camber of the work.
In recent years, along with higher speed and higher resolution of a precision apparatus such as a copying machine or a printer, there has been required a rail or the like which has more highly-accurate flatness and straightness. However, in the method described in Japanese Patent No. 3280733, the stress is gradually released by movements of the divided female molds, and thus a stress condition of the work varies considerably for each female mold which moves. Thus, the stress cannot be released continuously. In addition, due to mechanical sliding accompanying the movements of the female molds, variations in amount of camber are increased for each product. Moreover, the structure of the bending apparatus becomes complicated, leading to an increase in cost for an apparatus.
Further, in the method described in Japanese Patent Application Laid-Open No. 2004-074239, the amount of camber of the work can be suppressed, but the stress which is generated due to plastic deformation of the work increases. In addition, the method works locally, and hence a distribution of the stresses along the bending ridge line becomes extremely complicated, which incurs a risk that the work may be undulated or locally deformed by a large amount.